The invention relates to a method and apparatus for testing electronically discrete devices such as light-emitting diodes while still in the wafer form. Autoprobers, as the test machines are called, normally mark the defective electronic devices in some manner, usually with a solenoid operated inker of some variety, which are later discarded. The usual sequence of events is for a vacuum chuck on which the wafer to be tested is mounted, is raised up in the vertical or Z axis under the influence of a pneumatic cylinder or solenoid. The device to be tested, hereinafter referred to as a die, is electrically connected through a number of spring-loaded needle-like probes to an automatic test sequencer. A surface sensor also makes contact with the wafer surface, opens a microswitch and via digital logic circuitry provides the signal for the tester to begin its test sequence. Attached to the chuck normally is a metal flag which interrupts a beam of light between a light emitter and a detector when the chuck is in the up position to provide a second signal necessary to start the testing apparatus. Some probers only require the surface sensing, others require both surface and the chuck up signal prior to initiation of the test sequences. The test sequences are developed in an apparatus external to the autoprober and, through electrical connections to the probe support ring and the individual probes, are applied to the die whose response to the test sequence is also monitored and compared with known or desired responses. If a defective die is found, it is marked accordingly and discarded during the later processing which cuts up the wafer and separates the numerous dies printed on its surface.
Upon completion of the test series, an end-of-test (EOT) signal causes the vacuum chuck to drop to its lower limit allowing indexing in the X and Y axes to the next die printed on the wafer at which point the complete autoprober sequence is initiated again.
Many autoprobers have only a manual Z axis adjustment which requires an operator to monitor with percision the height of the vacuum chuck when it comes up. A problem if it is not up far enough or if it is up too high is that the inkers which are supposed to leave a 5, 10 or 15 mil. ink dot may leave a much smaller dot than they are supposed to or may obliterate the chip completely by leaving too large an ink dot. The result is that some defective dies might not be discarded because the ink dot is so small as not to be noticable or other dies adjacent to a defective die might also become covered with ink in the event that the ink dot is too large. An additional problem emphasizing the criticality of the Z axis adjustment is that it is desirable for the needle-ended probers to actually penetrate the contact surfaces slightly. In die construction it is common to use a vapor-deposited aluminum coating as an electrical connection contact; however, it is well known that an aluminum oxide coating can form on the outside of such a metallized contact making a low resistance contact extremely difficult. It is thus desirable for the needle point to penetrate the aluminum oxide to the non-oxidized aluminum coating without penetrating so far through that the complete die is destroyed. This then requires a very narrow operating range in the Z axis direction. Therefore, it is extremely desirable to maintain a precise Z axis orientation each time the chuck is in its pneumatic or solenoid operated up position.
In the past, numerous extremely complicated and expensive devices have been used to provide Z axis control such as a laser beam impinging upon the wafer and then the phase relationship of the reflected beam compared to the initial beam by a computer such that a precise vertical orientation is determined. Unfortunately, methods of this sort are extremely expensive and are economically prohibited from application in the smaller automatic and semi-automatic autoprobers. Therefore, many of the common prior-art autoprobers, such as the Electroglass model 1032 semi-automatic prober/inspector from Electroglass Inc. in Menlo Park, California, utilize a mechanical microswitch to sense the Z axis position of the surface of the wafer. The problems associated with such a mechanical switch are well known; for example, thermal effects, electronic noise, mechanical wear, etc. Therefore, elimination of the mechanical surface sensing device would allow a more accurate determination of the Z axis position.